1. Field of the Invention
The present invention relates to a phase synchronizing apparatus and a phase synchronizing method and is preferably applied to a disc drive for recording or reproducing data in or from a discoid recording medium such as a magnetic disc or an optical disc.
2. Description of the Related Art
The servo system for such a disc drive includes, for example, a servo-surface servo system for obtaining servo information from an exclusive servo surface, and an embedded servo system in which a data recording region and a servo information region are formed on the same disc surface in a time-division multiplex manner. To realize the above servo system, a disc drive is provided with a phase synchronizing circuit for generating a servo clock signal continuously synchronizing for one turn of a disc.
As shown in FIG. 1, a phase synchronizing circuit 1 is a so-called phase locked loop (PLL) circuit which consists of a phase comparator 2, a loop filter 3, a voltage control oscillator (VCO) 4, and a frequency divider 5.
A time reference signal S1 obtained by reproducing a clock mark in a servo region formed on the surface of a disc at a regular time interval is supplied to one input terminal of the phase comparator 2. Moreover, a clock signal SCLK output from the VCO 4 is supplied to the other input terminal of the phase comparator 2 via the frequency divider 5 after being frequency-divided into 1/N times (N: natural number). Thereby, the phase comparator 2 generates a voltage corresponding to the phase difference between the time reference signal S1 and the clock signal SCLK, and supplies the voltage to the loop filter 3 as a phase error signal S2.
The loop filer 3 applies a predetermined filtering processing such as low-pass filtering to the phase error signal S2 supplied from the phase comparator 2 and thereafter, supplies the processed signal to the VCO 4. The VCO 4 outputs a clock signal SCLK having a frequency and a phase corresponding to the voltage level of the phase error signal S2 obtained through the loop filter 3.
Thus, the phase synchronizing circuit 1 can output the clock signal SCLK having a frequency N times higher than that of the phase error signal S2 and synchronizing with the phase error signal S2 from the VCO 4, by feedback-controlling the phase difference between the phase error signal S2 and the clock signal SCLK so as to keep it constant.
However, in case where the phase synchronizing circuit 1 is applied to such as a variable-medium-type disc drive or a disc drive using a disc in which a phase synchronizing mark is previously embedded in a servo region when the disc is manufactured, it is very difficult to generate a high-accuracy clock signal because the following problems occur.
Firstly, when chucking a disc to a spindle, the effective circumferential speed fluctuates to a high speed at a portion where the substantial radius of a track circle is large, and to a low speed at a portion where it is small, due to an eccentricity in which the center of a rotary shaft and the center of a track circle are offset. Therefore, a problem occurs that a large phase fluctuation occurs in the time reference signal S1 reproduced from the track.
In case where, for example, twelve servo regions for one turn of a disc are formed at regular time intervals, a signal waveform of the time reference signal S1 obtained by reproducing a clock mark in each of the servo regions is shown in FIG. 2A. However, reproduced waveforms except for the clock mark, such as a position signal, track address, synchronizing pattern, and user data, are omitted.
In the time reference signal S1, the servo regions appear at regular time intervals shown by time t0-t12 in the case when the disc has no eccentricity. However, if the disc has an eccentricity, the time when the servo region appears is advanced or delayed. The phase-shift value of each servo region in the above case is shown as the arrow attached to the lower stage of the signal waveform of the time reference signal S1.
Therefore, in the time reference signal S1, time intervals for the servo region to appear become dense nearby the time t3 where a track circle has a large substantial radius, whereas they become nondense nearby the time t9 where the track circle has a small substantial radius.
FIG. 2B shows a graph obtained by arranging the phase-shift value of each servo region on the axis of ordinate. In this case, the sine-wave-shaped time reference signal S1 is input to the one input terminal of the phase comparator 2 in the phase synchronizing circuit 1 shown in FIG. 1 as described above.
The PLL of the phase synchronizing circuit 1 requires to accurately follow the phase-shift value of each of time t0-t9. However, since an open loop gain of passing through the phase comparator 2, the loop filter 3, the VCO 4 and the frequency divider 5, generally takes a limited value, a phase of the clock signal SCLK returned to the other input terminal of the phase comparator 2 has an amplitude slightly smaller than that of the time reference signal S1 and is shifted as shown in FIG. 2C.
Therefore, the eccentricity-following residue shown in FIG. 2D occurs in the phase error signal S2 output from the phase comparator 2. For example, in the case where a disc having a radius of 16 mm has an eccentricity of 120 xcexcm while rotating at 75 turns per second, the phase-shift value shows approximately 16 xcexcs. Even if the open loop gain of the PLL at 75 Hz is 60 dB, an eccentricity-following residue of 16 nanoseconds remains.
When the low-frequency gain of the PLL is increased in order to completely follow the phase fluctuation due to the eccentricity, high-frequency noises are increased due to band widening accompanying with the increase of the low-frequency gain, for example. Therefore, the problem occurs that it is very difficult to make the PLL accurately follow a disc eccentricity.
Secondly, to band-wide a PLL without increasing noises, a method of raising the phase comparison frequency by increasing the number of servo regions has been proposed. However, because a lot of time is required to switch over between recording and reproducing particularly in case of a magnetic disc, the utilization efficiency of the data surface lowers when the number of servo regions for one turn of the disc is increased. As a result, it is practically very difficult to form hundreds of servo regions for one turn of the disc.
To solve the above first and second problems, for example, a feedforward compensation method is disclosed (the U.S. Pat. No. 5,615,191) in which eccentricity errors of the phase and frequency of an output clock signal are corrected by previously measuring the eccentricity value of a disc, adding the eccentricity value to a PLL, and thereby making the phase and frequency follow the eccentricity value.
However, according to the aforementioned method, it is necessary to use an eccentricity measurement circuit for measuring the eccentricity value of a disc, and a measurement time equivalent to at least one turn of the disc is required for measurement of the eccentricity value. Moreover, the measured eccentricity value is temporarily stored in a memory. However, because a phase fluctuation value due to eccentricity increases toward inner tracks of the disc, and decreases toward outer tracks of the disc, a large-capacity memory and a large-scale circuit are required if the measured eccentricity value is stored over the entire disc radius to be sought by a head.
Moreover, according to the above-described feedforward compensation method using the open loop, characteristics of the PLL are changed due to temporal changes of element constants of the analog circuit section of such a loop filter and thereby, errors occur in the phase and frequency of an output clock signal. Therefore, the conventional compensation method is insufficient for practical use.
In view of the foregoing, an object of this invention is to provide a phase synchronizing apparatus, a phase synchronizing method and a disc drive capable of accurately following the eccentricity of a discoid recording medium.
The foregoing object and other objects of the invention have been achieved by the provision of a phase synchronizing apparatus for successively forming a servo region on a discoid recording medium every predetermined interval, obtaining a time reference signal by successively reading the servo regions from the discoid recording medium, and outputting a clock signal synchronizing with the phase of the time reference signal, the phase synchronizing apparatus comprises phase comparison means for outputting the phase difference between the time reference signal and a clock signal, clock signal generation means for generating a clock signal having a phase corresponding to the output of the phase comparison means and thereafter feeding back the clock signal to the phase comparison means, sine-wave-signal generation means for generating a plurality of sine-wave signals having phases different from each other based on a rotation synchronizing signal synchronizing with the rotation of the discoid recording medium, amplitude correction means for successively correcting amplitudes of the plurality of sine-wave signals obtained by the sine-wave-signal generation means in accordance with the output of the phase comparison means, and arithmetic processing means for synthesizing a plurality of sine-wave signals whose amplitudes are corrected by the amplitude correction means and adding them to the output of the phase comparison means.
Thus, by generating a plurality of sine-wave signals synchronizing with the rotation of the discoid recording medium and having phases different from each other, correcting the amplitudes of the sine waveforms successively in accordance with the output of the phase comparison means, and thereafter adding the correction result to a closed loop composed of the phase comparison means and clock-signal generation means, it is possible to correct a phase error produced in the output of the phase comparison means in accordance with a phase fluctuation even if the phase fluctuation occurs in a time reference signal due to the eccentricity value of the discoid recording medium.
Moreover, the present invention uses a phase synchronizing method of successively forming a servo region on a discoid recording medium every predetermined interval, obtaining a time reference signal by successively reading the servo region from the discoid recording medium, and outputting a clock signal synchronizing with the phase of the time reference signal, which comprises the steps of outputting the phase difference between a time reference signal and a clock signal with the phase comparison means, generating a clock signal having a phase corresponding to the output of the phase comparison means and thereafter, feeding back the clock signal to the phase comparison means, generating a plurality of sine-wave signals having phases different from each other with the sine-wave-signal generation means based on a rotation-synchronizing signal synchronizing with the rotation of the discoid recording medium, successively correcting the amplitudes of the plurality of sine-wave signals obtained from the sine-wave-signal generation means with amplitude correction means in accordance with the output of the phase comparison means, and synthesizing the plurality of sine-wave signals whose amplitudes are corrected by the amplitude correction means and adding them to the output of the phase comparison means.
Thus, by generating a plurality of sine-wave signals synchronizing with the rotation of the discoid recording medium and having phases different from each other, successively correcting the amplitudes of the sine waveforms in accordance with the output of the phase comparison means, and thereafter adding the correction result to a closed loop composed of the phase comparison means and the clock-signal generation means, it is possible to correct a phase error produced in the output of the phase comparison means in accordance with a phase fluctuation even if the phase fluctuation occurs in a time reference signal due to the eccentricity value of the discoid recording medium.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.